In order to play Tower Duel with a teammate, you will need to create or join an existing team. We invite you to visit our articles How do I create a team? and How do I join a team? for more information about the steps you will need to take.
If you are already part of a team, under the My Team tab, you will be able to see the list of members and their activity status, i.e. whether they are online or offline. Next to their names, you will see the amount of medals they have, their levels and a third option marked with the Tower Duel icon.
By clicking on the latter, you will be able to play a game of Tower Duel with the selected teammate, as long as they are online. For more information on this game mode, we recommend you visit the article How do I play Tower Duel?